How to Extend The Service Life of Lithium Batteries(II)

The capacity loss of lithium batteries is an extremely complicated process involving many factors. The capacity loss of lithium batteries can be divided into two parts: the capacity loss over time (calendar aging) (the lithium battery is left unused, the capacity will be reduced over time) and the capacity loss caused by use (cycle aging).

Regarding the calendar aging, the main factors involved are the State of Charge.
This refers to the amount of electricity in total capacity.
For example, it is 40% or 60%; temperature (Temperature); storage time (time).

Regarding the cycle aging, the main factors involved are:
Depth of Discharge each time. For example, each time you charge from 0% to 100%, and then discharge to 0%, or start charging when the battery is 20%, and unplug the battery when it reaches 80%. This is different; State of Charge (State of Charge), also known as electricity. For the same DOD, the average SOC can be different. For example, if the battery is kept between 40% and 100%, and kept between 20% and 80%, although the charge and discharge depth is the same, the impact on the battery is different due to the different charging state;

Charge rate. If the charging current can fully charge the battery within 1 hour, we say that the average charging rate is 1C; if the charging current can fully charge the battery within 2 hours, we say that the average charging rate is 0.5C; etc.; Temperature (Temperature );

Number of cycles. Obviously, cycling for two hundred cycles is lossier than one hundred cycles… In addition, there are some factors that are almost beyond our control. For example, a lithium battery in the initial stage of use will undergo a process of forming a solid electrolyte phase interface film (SEI film). This process consumes a certain amount of lithium ions. As long as the battery needs to be used, this process cannot be bypassed, so we don’t have to think about it too much.

Calendar aging and cycle aging are basically independent. Therefore, if the device can directly use an external power source without charging and discharging the lithium battery, then cycle aging can be avoided, which is beneficial to the life of the lithium battery. However, what SOC should we stay in? This is what will be discussed below: the qualitative law about the influence of various factors on the life loss of lithium batteries.

(1) State of Charge
Research shows that when the SOC is lower, both calendar aging and cycle aging will be delayed. Therefore, if we want to minimize the life decay of lithium batteries, we should keep their power low. For example, if you want the device to directly use an external power source without charging and discharging the lithium battery, it is better to keep the battery at 40% than at 60%. So, as long as the use requires permission, is the battery as low as possible? It depends on whether you let the lithium battery not participate in charging and discharging, such as storing it on hold or using only external power (only calendar aging at this time), or let it participate in charging and discharging (cycle aging is dominant at this time). In the former case, the power is indeed as low as possible. However, if the battery is too low, the risk of starving the battery is even greater if you forget to recharge it. Under the circumstance of ensuring that the battery will not starve to death, the protection effect is better if the battery is discharged to 5% or even close to 0% and then stored. At least at a power level above 40%, the internal resistance of the battery can be ignored. Therefore, for example, maintaining the power at 40% ± 20% is more beneficial than maintaining the power at 60% ± 20%.

(2) Temperature
What temperature is most friendly to lithium batteries? The data obtained from different studies are not exactly the same, but it is roughly similar to the comfortable temperature of the human body. So, keep the temperature at a room temperature that makes you comfortable. At higher temperatures (almost higher than a person’s normal oral temperature), the aging process is much faster anyway. At lower temperatures (almost 0°C), storage is basically no problem, but charging will cause more damage than usual. At very low temperatures (almost <20°C), even storage is not suitable.

(3) Depth of Discharge (DOD)
The shallower the depth of discharge, the better. It is much better to charge it for a short period of time several times a day than to charge it almost completely every day and then recharge it at night. You may have a question: If the charge is shallow, wouldn’t the number of cycles naturally increase? For example, if we can use 500 cycles according to 100% charge and discharge depth, don’t we of course expect to use 1000 cycles according to 50% depth? In fact, this is not the case. The researchers say that each cycle is based on the cumulative amount of charge and discharge reached 100%. Under this definition, we still get the conclusion that the shallower the charge and discharge, the better, that means that, for example, at a depth of 50%, you can count on 2000 charge and discharge times.

(4) Charge rate
The lower charge rate is better. If you are in no hurry, it is recommended to reduce the use of fast charging. However, the fast charge rate of digital products such as mobile phones and tablets is at most about 2C, which is far less than the 5C or even 15C that researchers found to be more harmful during the study. Therefore, the charging rate of these devices is a relatively small factor.

(5) Time and Number of cycles
The newer the battery and the less used it is, the less the capacity loss will of course be.

Summarize the above: Try to use lithium batteries and charge the battery at room temperature; choose a relatively low power level, keep the actual power above and below it, and avoid always being fully charged; shallow charge and shallow discharge, eat less and more meals.

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